Electromechanical relays and contactors are actuated to close contacts, open contacts, or to both close and open contacts, by one or more energized coils. Typically the coils are powered by a fixed voltage source. The strength of a magnetic field operating an electromechanical relay or contactor is a function of the product of the number of turns of the coil wire and the magnitude of the current, measured in amperes, passing through the coil wire. Coils have a fixed number of turns of coil wire, typically copper, that does not change after the coil is made. The only controllable variable in operating the coil is the magnitude of coil current. For a coil operated by a fixed direct current (DC) voltage source, the magnitude of coil current is determined by the voltage applied to the coil and the resistance of the coil. The coil resistance varies with the temperature of the coil exhibiting a positive temperature coefficient which causes the coil resistance to increase as the temperature of the coil wire increases and causes the coil resistance to decrease as the temperature of the coil wire decreases.
The coil in an electromechanical relay or contactor is typically powered by a fixed voltage source. When the temperature of the coil increases, due to heat generated when current passes through the coil during operation or due to a change in ambient temperature, the resistance of the coil increases and applications having a fixed voltage powering the coil provide less current to the coil which concomitantly results in producing a weaker magnetic field to actuate the coil of the relay or contactor. When the magnitude of current is too small, the coil may slowly actuate a relay or contactor causing an opportunity for prolonged arcing of contacts, welding of contacts, or in a worse case, the magnetic field may not be strong enough for the relay or contactor to actuate.
Conversely, when the coil temperature decreases, due to less heat being generated as a result of less frequent operation or a decrease in the ambient temperature, the resistance of the coil decreases and applications having a fixed voltage powering the coil provide more current to the cooler coil than when the coil is operated at a warmer temperature. In addition, when the magnitude of the current is too large, the relay or contactor will actuate more forcefully which can cause damage to contacts being subjected to excessive forces. The increased current can cause the undesirable condition of exceeding a power budget allocated to the system in which the electromechanical relay or contactor operates.
What is a needed is a technique to provide a constant magnitude current to produce a constant magnetic field to actuate the coil of electromechanical relays and contactors over a wide temperature range without providing a current of insufficient or excessive magnitude.